Basaltic lavas from Hainan Island near the northern edge of the South China Sea have an age range of between late Miocene (about 13 Ma) and Holocene, with a peak age of late Pliocene to middle ...Pleistocene. The basaltic province is dominated by tholeiites with subordinate alkali basalts. Most analysed samples display light rare earth element (LREE) enriched REE patterns and ocean island basalt (OIB)-like incompatible element distributions. The basalts contain abundant undeformed high-Mg olivine phenocrysts (up to Fo90·7) that are high in CaO and MnO, indicating high-magnesian parental magmas. Independent barometers indicate that clinopyroxenes in the basalts crystallized over a wide range of pressures of 2-25 kbar (dominantly at 10-15 kbar) and that the melt cooled from about 1350°C to 1100°C during their crystallization. The compositional characteristics of the basalts indicate that their generation most probably involved both low-silica and high-silica melts, as represented by the alkali basalts and tholeiites, respectively. Our results show that the source region for the Hainan basalts is highly heterogeneous. The source for the tholeiites is mainly composed of peridotite and recycled oceanic crust, whereas the source for the bulk of the low-Th alkali basalts consists predominantly of peridotite and low-silica eclogite (garnet pyroxenite). Some high-Th ( 4 ppm) alkali basalts may have been produced by partial melting of low-silica garnet pyroxenite (eclogite). We estimated the primary melt compositions for the Hainan basalts using the most forsteritic olivine (Fo90·7) composition and the most primitive bulk-rock samples (MgO > 9·0 wt % and CaO >8·0 wt %), assuming a constant Fe-Mg exchange partition coefficient of K
D = 0·31 and Fe3+/FeT = 0·1. The effective melting pressure (P
f) and melting temperature (T) of the primary melts are P
f = 18-32 kbar (weighted average = 23·8 ± 1·8 kbar) and T = 1420-1520°C for the tholeiites, and P
f = 25-32 (weighted average = 28·3 ± 1·4 kbar) and T = 1480-1530°C for the alkali basalts. The P
f -T data form an array that plots systematically above the dry lherzolite solidus but below the base of the lithosphere (∼55 km) and intersects the dry peridotite solidus at a pressure of about 50 kbar. The mantle potential temperature beneath Hainan Island, based on the estimate primary melt compositions, varies from about 1500 to 1580°C with a weighted average of 1541 ± 10°C. The high-magnesian olivine phenocrysts, high mantle potential temperature, and the presence of recycled oceanic crust in the source region provide independent support for the Hainan plume model that has previously been proposed largely based on geophysical observations. The Hainan plume thus provides a rare example of a young mantle plume associated with deep slab subduction.
The Gangbian alkaline complex in the southeastern Yangtze Block (South China) is composed of Si-undersaturated pyroxene syenites and Si-saturated to -oversaturated syenites and quartz monzonites. ...SIMS zircon U–Pb analyses indicate that the complex was emplaced at 848
±
4 Ma, during a previously-recognized interval of magmatic quiescence between the ca 1.0–0.89 Ga Sibaoan orogenic magmatism and the ca 0.83–0.78 Ga magmatic flare-up. The Gangbian rocks are characterized by wide, coherent variations in major and trace elements (SiO
2
=
47.6–68.4%, K
2O
+
Na
2O
=
4.5–10.5%, K
2O/Na
2O
=
0.4–1.2, MgO
=
1.2–8.5%, Cr
=
4.5–239 ppm, and Ni
=
4.5–143 ppm) and by enrichment in LIL and LREE and depletion in Nb, Ta and P in trace element spidergrams. Their whole-rock εNd(
T) (−
6.5 to −
0.4) and εHf(
T) (−
10.7 to 0.4) are positively correlated, suggesting involvement of both metasomatized mantle and continental crust materials in their genesis.
In situ zircon Hf–O isotopic measurements for the most evolved quartz monzonite sample yield a binary mixing trend between the mantle- and supracrustal-derived melts. It is suggested that the pyroxene syenites were derived by partial melting of metasomatized, phlogopite-bearing lithospheric mantle, and the parental magma experienced extensive fractionation of pyroxene and olivine associated with varying degrees of crustal contamination. Subsequent fractional crystallization of hornblende and minor amounts of plagioclase from the alkali basaltic magmas, accompanied by crustal contamination, produced the Si-saturated to -oversaturated syenites and quartz monzonites. These ca. 0.85 Ga alkaline rocks and neighboring contemporaneous dolerite dykes are the products of the anorogenic magmatism after the Sibao Orogeny. They post-date the final amalgamation between the Yangtze and Cathaysia Blocks, most likely manifesting the initial rifting of South China within the Rodinia supercontinent.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Whether or not mantle plumes and plate subduction are genetically linked is a fundamental geoscience question that impinges on our understanding of how the Earth works. Late Cenozoic basalts in ...Southeast Asia are globally unique in relation to this question because they occur above a seismically detected thermal plume adjacent to deep subducted slabs. In this study, we present new Pb, Sr, Nd, and Os isotope data for the Hainan flood basalts. Together with a compilation of published results, our work shows that less contaminated basaltic samples from the synchronous basaltic eruptions in Hainan–Leizhou peninsula, the Indochina peninsula and the South China Sea seamounts share the same isotopic and geochemical characteristics. They have FOZO-like Sr, Nd, and Pb isotopic compositions (the dominant lower mantle component). These basalts have primitive Pb isotopic compositions that lie on, or very close to, 4.5- to 4.4-Ga geochrons on 207Pb/204Pb versus 206Pb/204Pb diagram, suggesting a mantle source developed early in Earthʼs history (4.5–4.4 Ga). Furthermore, our detailed geochemical and Sr, Nd, Pb and Os isotopic analyses suggest the presence of 0.5–0.2 Ga recycled components in the late Cenozoic Hainan plume basalts. This implies a mantle circulation rate of >1 cm/yr, which is similar to that of previous estimates for the Hawaiian mantle plume. The identification of the ancient mantle reservoir and young recycled materials in the source region of these synchronous basalts is consistent with the seismically detected lower mantle-rooted Hainan plume that is adjacent to deep subducted slab-like seismic structures just above the core–mantle boundary. We speculate that the continued deep subduction and the presence of a dense segregated basaltic layer may have triggered the plume to rise from the thermal–chemical pile. This work therefore suggests a dynamic linkage between deep subduction and mantle plume generation.
•Late Cenozoic basalts in Southeast Asia display primitive Sr, Nd and Pb isotopes.•Coexistence of a 4.5–4.4 Ga-old reservoir and a minor young recycled component.•Support dynamic linkages between deep subduction and mantle plume generation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•Elevated Th/U and diminished Rb/Sr ratios serve as reliable indicators for discerning water-fluxed melting.•The occurrence of water-fluxed melting in the Himalayas is substantiated ...through rigorous thermodynamic modeling.•The liberation of free water can be traced to both dehydration melting and prograde metamorphism, underlining their roles as water release mechanisms.
Knowledge of the role water has played in the evolution of Earth’s continents is crucial for improving our understanding of how its layers (e.g. crust and mantle) interact with each other and co-evolved. Orogenic belts are the preferred pathway for material circulation, heat transfer, and water cycling. The Himalayan orogen preserves a large number of syn- and post-collisional rocks, of which the Himalayan leucogranites are key to studying the role of water in crustal reworking. Although some geochemical proxies support water-fluxed melting in the source of Himalayan leucogranites, the timing of water addition, the scale of water-fluxed melting, and how water affects or controls the chemical and physical properties of the partial melts are not well constrained. Here, we filtered the published data on High Himalayan leucogranites to ensure that they represent the geochemical characteristics of primary melts. Our results show that the primary leucogranites with high Th/U and low Rb/Sr ratios were produced by water-fluxed melting as a result of the dissolution of monazite, melting of plagioclase, and formation of peritectic K-feldspar. Furthermore, thermodynamic modeling demonstrates that water-fluxed melting generates a high water content and a large volume of felsic water-unsaturated melt with chemical compositions similar to those of the High Himalayan leucogranites. We suggest that the external water is released by the breakdown of hydrous crustal minerals during dehydration melting and prograde metamorphism.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Early Jurassic bimodal volcanic and intrusive rocks in southern South China show distinct associations and distribution patterns in comparison with those of the Middle Jurassic and Cretaceous rocks ...in the area. It is widely accepted that these rocks formed in an extensional setting, although the timing of the onset and the tectonic driver for extension are debated. Here, we present systematic LA–ICP–MS zircon U–Pb ages, whole-rock geochemistry and Sr–Nd isotope data for bimodal volcanic rocks from the Changpu Formation in the Changpu–Baimianshi and Dongkeng–Linjiang basins in southern Jiangxi Province, South China. Zircon U–Pb ages indicate that the bimodal volcanic rocks erupted at ca. 190Ma, contemporaneous with the Fankeng basalts (~183Ma). A compilation of geochronological results demonstrates that basin-scale basaltic eruptions occurred during the Early Jurassic within a relatively short interval (<5Ma). These Early Jurassic basalts have tholeiitic compositions and OIB-like trace element distribution patterns. Geochemical analyses show that the basalts were derived from depleted asthenospheric mantle, dominated by a volatile-free peridotite source. The calculated primary melt compositions suggest that the basalts formed at 1.9–2.1GPa, with melting temperatures of 1378°C–1405°C and a mantle potential temperature (TP) ranging from 1383°C to 1407°C. The temperature range is somewhat hotter than normal mid-ocean-basalt (MORB) mantle but similar to an intra-plate continental mantle setting, such as the Basin and Range Province in western North America. This study provides an important constraint on the Early Jurassic mantle thermal state beneath South China.
•Bimodal volcanic rocks from southern Jiangxi Province erupted at ca. 190Ma.•The basalts were derived from the asthenospheric mantle.•Numerical simulation results constrain a partial melting pressure of 1.9–2.1GPa and an average mantle potential temperature at 1400°C.•A Basin and Range-type lithosphere existed in South China during the Early Jurassic.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•The protoliths of Neoproterozoic gneisses in the eastern Chinese Central Tianshan Block (CTB) are S-type granites.•The basement of the CTB formed in the early Mesoproterozoic and ...reworked at Neoproterozoic.•The Mesoproterozoic crustal growth was related to the Nuna breakup.•The early Neoproterozoic crustal reworking associated with the Rodinia assembly.•The CTB has no close tectonic affinity to the Tarim Craton in the Precambrian.
As the southernmost continental fragment in the Central Asian Orogenic Belt (CAOB), the Chinese Central Tianshan Block (CTB) is essential for understanding the evolution of the CAOB. However, its tectonic affinity with the Tarim Craton and links with supercontinent cycles are not clear. Here, we present whole-rock geochemistry, zircon ages and Hf-in-zircon isotopic data for augen- and mylonitic granitic gneisses in the eastern Chinese Central Tianshan Block (ECTB). Zircon U-Pb dating reveals that the augen- and mylonitic gneisses formed at ca. 918Ma and 896Ma, respectively. The gneisses have REE and HFSEs patterns comparable to the upper continental crust. Their Cr and Ni contents are similar to those of the typical S-type granites in the Lachlan belt. These rocks exhibit evolved zircon εHf(t) values (−9.0 to +1.6), which are consistent with those values of coeval crustal-derived rocks within the CTB. Together with the occurrence of muscovite and the existences of Paleo- to Mesoproterozoic inherited zircons (2.21–1.25Ga), the geochemical data indicate that protoliths of these gneisses are S-type granites. These results, compiled with published geochronological data, suggest that an Archean basement was most likely absent in the CTB. The basement rocks of the CTB were dominantly produced by crustal growth in the early Mesoproterozoic and then reworked at Neoproterozoic. We suggest that the Mesoproterozoic crustal growth and the early Neoproterozoic crustal reworking were likely related to the breakup of the Nuna (ca. 1.40Ga) and the assembly of the Rodinia (1.00–0.88Ga), respectively. Because the CTB displays different crustal evolution from the Tarim Craton, we conclude that these two blocks have no close tectonic affinity in the Precambrian.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
A straightforward tandem column separation procedure is presented for the separation of Sr, Nd, Pb, and Hf from silicate materials. It allows rapid purification, without any intervening evaporation, ...of these four elements of great interest in Earth science and cosmochemistry. After sample loading, the upper Sr Spec column adsorbs Sr and Pb, while the lower TODGA Spec column adsorbs Hf and Nd. Strontium-lead and hafnium-neodymium elements are then back-extracted from the Sr Spec and TODGA Spec columns, respectively. The whole separation procedure, including column setup, cleaning, and pre-conditioning, takes approximately eight hours for separating a batch of 25 samples. The proposed procedure offers significant improvement in separation efficiency of these often-used four elements, compared with conventional four column methods. Fractions of Sr, Nd and Pb are then measured by TIMS and the Hf fraction is determined by MC-ICP-MS. The stability of this procedure was demonstrated by replicate measurements of
87
Sr/
86
Sr,
143
Nd/
144
Nd,
176
Hf/
177
Hf,
206
Pb/
204
Pb,
207
Pb/
204
Pb and
208
Pb/
204
Pb isotope ratios of eight international silicate rock reference materials, spanning a wide range of bulk compositions. The analytical results obtained in this study agree well with published data. The external reproducibility (2RSD,
n
= 8) of standard BCR-2 was ±0.0026% for
87
Sr/
86
Sr, ±0.0020% for
143
Nd/
144
Nd, ±0.0049% for
176
Hf/
177
Hf, and ±0.026-0.034% for
206
Pb/
204
Pb,
207
Pb/
204
Pb and
208
Pb/
204
Pb isotope ratios.
A straightforward tandem column separation procedure is presented for the separation of Sr, Nd, Pb, and Hf from silicate materials.
•This is the first to identify Mesoarchean (3.11–2.85 Ga) rocks in the southern Yangtze Block.•1.94 and 1.89 Ga granitic magmatism recorded in the Cuoke complex.•Multistage crustal growth and ...reworking occurred in the Cuoke complex.•The Yangtze Block experienced intense metamorphism and magmatism in the context of Nuna assembly.
The newly identified Cuoke Complex preserves voluminous early Precambrian granitoids, bearing key information for the early evolution of the Yangtze Block, South China. Here, we report new field observations, petrology, zircon U-Pb-Hf isotopes and trace elements for eight granitoids from this complex. The analyzed zircon domains are of magmatic origin, as evidenced by their oscillatory zoning, high Th/U ratios and magmatic rare earth element patterns. Three trondhjemitic gneisses yield crystallization ages of 3.11–3.06 Ga, documenting the oldest known rocks in the southern Yangtze Block. Their εHf(t) values vary from −3.2 to + 2.6 with depleted mantle two-stage (TDM2) model ages of 3.6–3.3 Ga, indicating reworking of Paleoarchean crustal materials. Three gneissic granitoids crystallized at 2.86–2.85 Ga and reveal a significant episode of granitoid magmatism in the late Mesoarchean. They possess positive εHf(t) values (+0.2 to +5.0) and slightly older TDM2 model ages (3.3–3.0 Ga), suggesting that these granitoids were most likely derived from remelting of juvenile continental crust. Two monzogranites with crystallization ages of 1.94 and 1.89 Ga display negative εHf(t) values (−13.0 to −8.1) and older TDM2 model ages (3.3–3.1 Ga), indicating substantial involvement of ancient crustal materials. Available zircon U-Pb-Hf isotopic data from the Cuoke Complex have revealed that the earliest crustal growth probably occurred in the early Paleoarchean, with significant crustal growth and reworking during the early Mesoarchean, followed by multistage crustal reworking during the late Mesoarchean to Paleoproterozoic. Integrating the results of this study with records from other Archean basement complexes in the Yangtze Block (i.e., Kongling, Zhongxiang, Douling, Yudongzi and Phan Si Pan), we propose that these complexes have diverse formation histories and generally experienced tectonothermal events at ca. 2.0–1.9 Ga, probably responding to the global-scale assembly of the Nuna supercontinent.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Thermal evolution of biomarkers trapped by asphaltene structure is distinctly delayed compared with that of free biomarkers.•The potential chemical conversion between asphaltene-occluded n-alkanes ...and n-alkenes may be related to the different metastable equilibrium of the system.•Steranes and terpanes occluded by asphaltenes can preserve the original characteristics of the diagenesis stage during thermal maturation.•The occlusion space formed by asphaltene aggregates presumably restricts the transformation of active biomarkers during the thermal evolution.
Asphaltene-trapped (adsorbed or occluded) biomarkers are considered a valid source of information for crude oil with severe secondary alterations. However, thermal stress might change asphaltene-trapped hydrocarbons to some extent. Therefore, it is indispensable to investigate the thermal evolution behavior of asphaltene-trapped biomarkers. In this study, low-maturity bitumen from the Kuangshanliang area of the Sichuan Basin (China) was selected as a sample. The thermal evolution behavior of free and asphaltene-trapped saturated biomarkers was investigated by thermal simulation experiments. The results revealed, in addition to normal biomarkers, also a series of even carbon number n-alk-(1)-enes in asphaltene-occluded hydrocarbons. All of them were hardly influenced by secondary alterations. Moreover, due to the restriction of macromolecular structure, the thermal evolution of asphaltene-trapped biomarkers lags behind that of free biomarkers. Most biomarker parameters of occluded hydrocarbons changed little with increasing maturity. In particular, the distributions of occluded terpanes or steranes retained the characteristics of early diagenesis of organic matter, even if they suffered from strong thermal alteration.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Felsic volcanic rocks from the Bikou Group show I– and A–type granite affinities.•These felsic volcanics were formed by partial melting of the juvenile crust, with involvement of various proportions ...of basement rocks.•The formation of the Bikou bimodal volcanic rocks responded to the breakup of Rodinia.
The petrogenesis of Neoproterozoic igneous rocks at the northwestern margin of the Yangtze Block and their tectonic setting are controversial. Here we present whole–rock major and trace elements and Nd–Hf isotopic data for felsic volcanic rocks from the Bikou Group. These ca. 820–810 Ma volcanic rocks consist of dacite and rhyolite with SiO2 ranging from 64.4 to 77.0 wt%. Most rocks show decreasing P2O5 with increasing SiO2 and Th and have I–type granite affinities (metaluminous volcanic rocks). However, four rhyolites from the lower part of the Bikou Group show A–type granite affinities (alkaline rhyolites), characterized by high 10,000*Ga/Al and FeOt/(FeOt + MgO) ratios and high (Zr + Nb + Ce + Y) contents. The felsic volcanic rocks commonly have relatively high Zr saturation temperature (up to 960 °C) and most of the felsic volcanic rocks have εNd(t) and εHf(t) values lower than the contemporaneous basalts in the Bikou Group, but higher than the basement rocks in the study area. Among them, four alkaline rhyolites have εNd(t) and εHf(t) values higher than the metaluminous volcanic rocks in the lower Bikou Group. We propose that the metaluminous volcanic rocks were likely produced by partial melting of newly underplated basaltic crust, with involvement of different proportions of basement rocks, whereas a two–stage model is proposed for the formation of the alkaline rhyolites, i.e. partial melting of newly underplated basaltic crust forming early granitoids, followed by low pressure partial melting of these granitoids to generate the alkaline rhyolitic magma. Combined with previous studies, the bimodal occurrence of volcanic rocks within the Bikou Group, and the A–type granite affinity of some rhyolites, as well as the presence of ocean island (OIB)–type basalts in the upper part of Bikou Group, suggest an extensional setting. This is consistent with opening of the Kangdian rift basin to the southwest at this time. We thus propose that the ca. 820–810 Ma Bikou bimodal volcanic rocks formed in response to the breakup of the Rodinia supercontinent.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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